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853S54I-01 Datasheet(PDF) 9 Page - Integrated Device Technology |
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853S54I-01 Datasheet(HTML) 9 Page - Integrated Device Technology |
9 / 20 page 9 ©2017 Integrated Device Technology, Inc. June 16, 2017 853S54I-01 Datasheet Applications Information Wiring the Differential Input to Accept Single-Ended Levels Figure 1 shows how a differential input can be wired to accept single ended levels. The reference voltage VREF = VCC/2 is generated by the bias resistors R1 and R2. The bypass capacitor (C1) is used to help filter noise on the DC bias. This bias circuit should be located as close to the input pin as possible. The ratio of R1 and R2 might need to be adjusted to position the VREF in the center of the input voltage swing. For example, if the input clock swing is 2.5V and VCC = 3.3V, R1 and R2 value should be adjusted to set VREF at 1.25V. The values below are for when both the single ended swing and VCC are at the same voltage. This configuration requires that the sum of the output impedance of the driver (Ro) and the series resistance (Rs) equals the transmission line impedance. In addition, matched termination at the input will attenuate the signal in half. This can be done in one of two ways. First, R3 and R4 in parallel should equal the transmission line impedance. For most 50 applications, R3 and R4 can be 100. The values of the resistors can be increased to reduce the loading for slower and weaker LVCMOS driver. When using single-ended signaling, the noise rejection benefits of differential signaling are reduced. Even though the differential input can handle full rail LVCMOS signaling, it is recommended that the amplitude be reduced. The datasheet specifies a lower differential amplitude, however this only applies to differential signals. For single-ended applications, the swing can be larger, however VIL cannot be less than -0.3V and VIH cannot be more than VCC + 0.3V. Though some of the recommended components might not be used, the pads should be placed in the layout. They can be utilized for debugging purposes. The datasheet specifications are characterized and guaranteed by using a differential signal. Figure 1. Recommended Schematic for Wiring a Differential Input to Accept Single-ended Levels Recommendations for Unused Input and Output Pins Inputs: IN/nIN Inputs For applications not requiring the use of the differential input, both INx and nINx can be left floating. Though not required, but for additional protection, a 1k resistor can be tied from INx to ground. LVCMOS Control Pins All control pins have internal pulldowns; additional resistance is not required but can be added for additional protection. A 1k resistor can be used. Outputs: LVPECL Outputs All unused LVPECL outputs can be left floating. We recommend that there is no trace attached. Both sides of the differential output pair should either be left floating or terminated. |
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